The 1970 CJ-5 Renegade came with a late version of the proven Spicer/Dana Model 18 transfer case, which was introduced in the original Willys MB in 1941. It was the heart of the Jeep four-wheel drive system and was integral to the "Go Anywhere, Do Anything" capability of the Jeep. The Model 18 served dutifully in a variety of Jeep vehicles for over 30 years; it was also used in International Harvester Scout 4x4s. It became legendary due to it's strength, durability, ease of service, and ability to easily handle V8 power.
The Spicer/Dana Model 18 is a cast iron, gear driven, offset-drive (meaning that both front and rear driveshafts are in-line) transfer case with dual modes (2wd and 4wd) and dual ranges (High and Low). The rear of the case offers a PTO port that can be used to power aftermarket accessories from winches to farm implements, or it can accommodate popular aftermarket overdrive units such as those from Warn, Saturn, and Husky. The Model 18 was offered in both single and twin-stick versions. A common modification for the twin-stick version is to remove the interlock pin, which allows for the added function of 2wd/Low. The fact that both the front and rear driveshafts are inline means that this transfer case must be paired with axles that have the differential offset to the passenger side.
If the Model 18 has one shortcoming, it would be the relatively high (numerically low) low-range gear ratio of 2.46:1. While very respectable, it falls short of the 2.72:1 to 4.0:1 ratios common to late model Jeeps. This can be corrected, if desired, by the installation of aftermarket low-range gear sets.
There are four civilian iterations of the Model 18 transfer-case. Another of the benefits of selecting the 1970 Renegade as the starting point for my project is that it was equipped with the final civilian version of the Model 18, which benefited from numerous upgrades and enhancements. Often referred to as the "large case" version of the Model 18, this version was only found behind the Dauntless V6 engine with its T86 and T14 transmissions. The case housing was essentially the same as its successor, the Model 20, which boasted a slightly larger case made with improved casting methods. This version employed a 1 1/4" intermediate shaft, the largest used in the Model 18, which allowed for increased bearing area. For this final version, the locating bore was increased from 3 5/32" to 4". A new front output cap was used which employed a single-stick shift mechanism. I like the added versatility that comes with the addition of 2wd/Low-range, so I swapped in the front output cap and twin-stick shift mechanism from the prior version of the Model 18.
I began with disassembly as captured in the pictures below. The process is fairly straightforward although it's worth noting that, as with disassembly of most automotive subsystems containing multiple small parts, it's highly recommended that you separate, bag and label the parts for future reference. I can't be the only one who has rationalized not taking the time to do so by saying, "I don't need to worry about that... I'm going to reassemble it tomorrow." Then several weeks later I'm staring down at a pile of parts thinking, "Now where the heck did this go?" Best intentions notwithstanding, save yourself the headache! I like to have a diagram or service manual handy so that I can label the parts with the correct name - it's extremely helpful to have consistent terminology between the instructions and my labeled parts. In this case, Novak Conversions offers an excellent Rebuilders Guide for the Model 18 complete with an exploded parts diagram.
One item of note: be aware that the quality of the intermediate shaft is critical. Due to the in-line design of the Model 18, this shaft is not only constantly spinning, but it is under constant load. Some of the name brand rebuild kits from the common 4x4 retailers contain cheaper, foreign made intermediate shafts of unsuitable alloys that lack the required hardness. Novak Conversions offers a super-durable shaft made from triple alloy gear steel that will ensure long life for your Model 18 rebuild. In the two intermediate shaft pictures below, you can see the detail of the wear on the cheaper intermediate shaft.
Additionally, you can see the earlier style front output cap with twin-stick shift provision as I reinstall it on the main case. Note the shift fork bolts - they are square head bolts with a small hole drilled in the head. This is provided to allow the bolts to be wired to prevent them from coming loose. There are numerous areas where Hi-Tack or a similar gasket sealer are required during reassembly.
You can see where the roller bearings are installed in their bore. This can be a little tricky and requires a gentle touch since the bearings are "free" and not caged, and they must be installed prior to sliding the intermediate shaft into place. In the pictures below, you can see the case with the main gears reinstalled. The subsequent pictures show the painted transfer-case once completed, including the drum brake backing plate and the pivot arm that the parking brake cable attaches to.
If you're questioning my choice of paint for the transfer-case (and the transmission), there's a practical reason for it and it has nothing to do with "bling". I selected the Seymour Hi-Tech Engine Coating for it's durability, but also because the brightness of the chrome metallic finish will make it easier to identify the source of any leaks. The underside of a Jeep gets plenty dark and dirty on it's own - the metallic finish will reflect light and make service and maintenance a little easier.
As mentioned on the Transmission page, I utilized an SM420 transmission to Model 18 t-case adapter from Novak Conversions. While not inexpensive, the adapter is machined from the highest quality billet aluminum with tight tolerances to make this swap seamless. It's really a thing of engineering beauty!
UPDATE: After completing this rebuild, I changed direction slightly and decided against the transfer-case-mounted parking brake. It seems that a lot of early Jeep owners report issues with this setup - frequent required adjustments, dirt and mud getting packed into the drum, ineffective braking action when wet, etc. You can read about the alternative parking brake I selected in the Brakes section.
In the pictures below, you'll see that the threaded mounting hole for the speedometer cable was cracked and a small piece was missing. This was addressed with strategically placed epoxy putty that hardens like steel.
As I did with the axles, I removed the OEM check valve from the Model 18 case and installed a threaded barb to enable attachment of a rubber hose and remotely mounted check valve higher up on the body. It's not unusual when fording a stream to have water up to the top of the transfer case. Again, it's a little extra insurance.